In order to understand a developmental switch with which phage lambda decides its fate, lytic growth or lysogeny, we wish to study in vitro the regulatory proteins involved. The positive regulator cII gene will be cloned into a plasmid to construct an over-producer strain. Having this, cII protein will be purified by conventional chromatographic procedures and characterized for its ability to bind DNA, interact with RNA polymerase, and initiate or terminate RNA synthesis in vitro. CIII protein and other host components involved will also be isolated using an affinity column and characterized for their roles. Action of Cro regulatory protein will be further explored in terms of operator recognition. Genetic experiments and chemical modifications of Cro repressor will be carried out to know the functional groups involved in specific and non-specific DNA binding. Besides, the core-sequence of lambda operator will be chemically synthesized, co-crystallized with Cro repressor which will be served for X-ray crystallographic analysis. All these informations will be used to make a molecular model of repressor-operator interaction.